Terahertz Astronomy: 1st Edition (Paperback) book cover

Terahertz Astronomy

1st Edition

By Christopher K. Walker

CRC Press

351 pages

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A Powerful Window into Cosmic Evolution

Terahertz (THz) observations of interstellar atoms, molecules, and dust serve as powerful probes of the conditions within the interstellar medium that permeates our galaxy, providing insights into the origins of stars, planets, galaxies, and the Universe. Taking a cross-disciplinary approach to the subject, Terahertz Astronomy explores THz astrophysics and the technologies that make this rapidly evolving field possible.

The first four chapters of the book discuss the origin and interpretation of THz light in astrophysical sources. The remaining five chapters present an overview of the technologies used to collect and detect THz light. Every chapter contains worked-out examples and exercises. The author explains each topic as intuitively as possible and includes the equations needed for real-life astrophysical applications.

In just a few years, the number of active THz researchers has substantially grown due to increased interest in terrestrial remote sensing at THz frequencies. This book provides researchers with both the background science and technology to interpret THz observations and design, build, and deploy THz astronomical instrumentation.


"The book is beautifully illustrated, having a long list of references and worked examples at the end of each chapter. I would recommend the book to students and researchers working in THz technologies and astronomy."

Optics & Photonics News, January 2016

"This comprehensive, well-written book was a pleasure to read, an absolute must for anyone interested in learning about cutting-edge technologies and techniques revolutionizing our modern view of the Universe, and it is destined to become a standard reference work for astronomers and technologists."

—Professor Glenn White, The Open University

"This excellent book fills a huge hole in the literature in observational terahertz astronomy. No other existing text covers the essentials of terahertz astronomy instrumentation and techniques as applied to the study of the interstellar medium. It is sure to become an essential text for those who work in the field."

—Christopher Groppi, School of Earth and Space Exploration, Arizona State University

"… a wonderful book, combining an insightful overview of the interstellar medium with a description of the most important techniques used to study this component of galaxies and how new stars form. It’s particularly timely, with the availability of powerful new tools, including the ALMA array, the SOFIA airborne observatory, and a range of planned and proposed suborbital and space missions in the terahertz range."

—Paul F. Goldsmith, Jet Propulsion Laboratory, California Institute of Technology and Herschel Space Observatory

"This valuable and well-written textbook by Professor Christopher Walker helps to fill a gap in the literature describing the interstellar medium at terahertz frequencies. There are very few textbooks that discuss the medium from the observer’s point of view, showing how to determine its physical parameters from the output of the detectors. Also discussed are the operations of mixer and bolometer detectors."

—Thomas G. Phillips, John D. MacArthur Professor of Physics Emeritus, California Institute of Technology

"… a fundamental source for astronomers and engineers who wish to understand the physical principles and detection techniques."

—Jonathan Williams, University of Hawaii

"Walker’s book is incredibly timely: it introduces the basics of terahertz data but also explains the instrumentation at the level all observers need to know, and does so in friendly but, most importantly, crystal-clear prose."

—George H. Rieke, Regents' Professor of Astronomy, University of Arizona

"… an essential resource for all astrophysicists and experimental physicists seeking to exploit the opportunities of this last frontier in astronomy."

—Michael Burton, University of New South Wales

"The first four chapters that cover stellar evolution, atomic physics and radiation physics are the basics…. The last five are… crucial as THz data are highly dependent on the observation and calibration techniques, especially the detector technology, the improvement of which is the reason why the field has moved forward the way it is… the examples and exercises will help the readers along the way."

Contemporary Physics (Aug 2016)

Table of Contents

The Interstellar Medium at Terahertz Frequencies


ISM Components of the Milky Way

Lifecycle of the ISM

Probing the Lifecycle of the ISM



THz Radiative Transfer Basics and Line Radiation

Equation of Radiative Transfer

Solution to the Equation of Radiative Transfer under LTE (Local Thermodynamic Equilibrium)

Radiative Transfer of Rotational Transitions of Linear Molecules in LTE

Non-LTE Approach


THz Continuum Emission


THz Spectral Energy Distributions

Deriving Dust Optical Depth, Column Density, and Mass

Temperature and Density Distributions

Dust Energy Balance in Clouds

Dust–Gas Coupling

Dust Polarization: Origin and Measurement


Simple Radiative Transfer Model



Source Physical Conditions

Lines of Sight

Model Equation of Transfer

LTE Radiative Transfer with Hydrodynamic Simulations

Non-LTE Radiative Transfer with Hydrodynamic Simulations

Going Further

THz Optical Systems

Introduction: Source-Beam Coupling

QED and Maxwell

Origin of a Single-Aperture Diffraction Pattern (QED)

Gaussian Beam Optics

Focusing Gaussian Beams

Intercepting Gaussian Beams

Illuminating THz Telescopes


THz Coherent Detection Systems


Superheterodyne Receivers

Receiver Noise Temperature

Noise Temperature of THz Optical Systems

THz Mixer Architectures and Noise

IF Amplifiers

Effective Temperature Measurement of THz Components

THz Mixers

THz Local Oscillators

Receiver Back-Ends

Receiver Stability and Allan Time

Heterodyne Array Considerations


Incoherent Detectors


Bolometer Basics

Semiconductor Bolometers

Superconducting Incoherent

Background Noise Limited Operation

Instrument Noise Limited Operation

Sensitivity Requirements

Comparing Heterodyne and Incoherent Detector Sensitivity

Incoherent Array Considerations


THz Observing Techniques


Observing Strategies

Receiver Calibration

Estimating Atmospheric Optical Depth

THz Brightness Temperature of Planetary Bodies


THz Interferometry


Simple Adding Interferometer

Phase Switched Interferometer

Correlation Interferometer

Phasor Equation for Interferometry

Aperture Synthesis of Extended Sources

Filling-in uv Space

Transforming the Visibility Function

Map Noise Level

Phase Closure or Self-Calibration

Phase Error at THz Frequencies

Answers to Problems

Appendix 1: Timeline of Thz Technology

Appendix 2: More Thz Transitions of Atoms and Molecules

Appendix 3: Commonly Used Physical and Astronomical Quantities

Appendix 4: Useful Radiative Transfer Expressions

Appendix 5: Commonly Used Quasi-Optical Expressions

Appendix 6: Useful Heterodyne Receiver Expressions

References appear at the end of each chapter.

About the Author

Christopher K. Walker is a Professor of Astronomy and an Associate Professor of Optical Sciences and Electrical & Computer Engineering at the University of Arizona. He has over 30 years of experience designing, building, and using state-of-the-art receiver systems for THz astronomy. He is the author of numerous papers on star formation and protostellar evolution, a topical editor for IEEE Transactions on TeraHertz Science and Technology, and the principal investigator of the long-duration balloon project "The Stratospheric THz Observatory (STO)" funded by NASA.

Subject Categories

BISAC Subject Codes/Headings:
SCIENCE / Astrophysics & Space Science
SCIENCE / Physics